Food products, pharmaceuticals, etc. need inspection for contaminants. In the process of manufacturing these products, for example, contaminants such as metal fragments may get mixed in the products. In order to ensure the safety of the products, it is necessary to eliminate metallic contaminants from them and hence essential to detect the metallic contaminants.
Stainless steel is excellent in corrosion resistance and widely used in various products, mainly in the fields of kitchen utensils, building materials, domestic electrical equipment, automotive parts, dairy and fermentation tanks, chemical equipment, thermal insulating apparatus, etc. There are many stainless-steel products particularly in the field of food products, such as food product containers, and structural components, containers and so forth of all machines in food product manufacturing lines and food product manufacturing machines. It is impossible to avoid wear particles or fragments of such stainless steel products getting mixed in food products. Stainless steels are alloy steels exhibiting excellent corrosion resistance.
Stainless steels are roughly divided into austenite stainless steels, martensite stainless steels, and ferrite stainless steels. It is known that these stainless steels exhibit magnetic properties. Therefore, various kinds of stainless steel can be detected as metallic contaminants by using their characteristic properties.
Conventional Detection Technique:
A sensor coil comprising a core and a copper wire wound around the core has heretofore frequently been used to detect a metallic contaminant. That is, a magnetic field is generated by the sensor coil, and an influence on the magnetic field exerted by a metallic substance when passing through the magnetic field is sensed to detect the metallic substance. More specifically, the conventional sensor coil comprises a magnetic field generating coil for generating a magnetic field and a magnetic field receiving coil for receiving the magnetic field. The magnetic field generating coil and the magnetic field receiving coil are disposed to face each other, and an inspection object is passed between them to detect a possible metallic contaminant.
The magnetic field receiving coil receives a magnetic field induced by an eddy current flowing through a metallic substance. For the receiving coil, it is easy to receive a large magnetic field. FIG. 6 shows a B-H characteristic graph (“magnetic flux density” to “magnetic field”), which is the magnetic history curve (magnetization intensity-magnetizing force) of the core constituting the receiving coil. It is general practice to use the linear region P2 or the relaxation region P3 of the magnetic history curve for the detection of a metallic contaminant by use of the above-described magnetic field generating coil.
A strong magnetic field produces a large magnetic flux. This causes a large eddy current to flow through a metallic substance contained in the inspection object. Thus, a large magnetic field induced by the eddy current is received by the magnetic field receiving coil. In many cases, the above-described magnetic field generating coil transmits an electromagnetic wave in the frequency band of 1 MHz to 333 kHz to the inspection object, and the magnetic field receiving coil, which is installed at the opposite side of the inspection object, detects the electromagnetic wave to identify a metallic contaminant.
The electromagnetic wave consists of a component passing through the inspection object; a component reflected by the inspection object; and a component absorbed by the inspection object. When the inspection object is small in size, the detection sensitivity is increased by generating as a strong electromagnetic wave as possible. For this purpose, the frequency of the electromagnetic wave may be increased. As the frequency becomes higher, the proportion of the reflective component increases. This may cause the reception sensitivity to be degraded, conversely.
Many of food products, pharmaceuticals, etc. are packaged in the manufacturing process and shipped in the packaged state. Packages are made of paper, aluminum and other various kinds of materials. However, it has been difficult to detect metallic contaminants in food products in packaging bags or containers made of electrically conductive materials, such as aluminum pouches or films, by using the conventional sensor coil. The reason for this is that an eddy current is produced in the containers as the magnetic field becomes strong, thus making it difficult to detect metallic contaminants in the containers.
For this reason, an X-ray apparatus is used to detect metallic contaminants in packaging bags or containers. An inspection object is irradiated with X-rays by the X-ray apparatus, and X-rays passing through the inspection object are received by a film or a detector for image projection placed at the opposite side of the inspection object. The received image is processed to judge the presence of a metallic contaminant. In other words, strong X-rays are passed through a packaging bag made of an electrically conductive material, and the transmitted X-rays are received. Then, image data obtained from the received X-rays is subjected to image processing to detect and identify a metallic contaminant.
This detection method suffers unfavorably low detection accuracy (the method can detect only bolts or nuts, for example, which have a diameter not less than about 6 millimeters or 8 millimeters). In addition, there is a danger that the operator of the X-ray apparatus may be exposed to X-rays. There are many other problems. That is, the X-ray apparatus is large in size and costs a great deal to maintain. In the case of a packaging bag having a food product or the like sealed therein, the end and central portions of the bag are different in thickness from each other. Consequently, the amount of X-rays transmitted differs according to the thickness. Accordingly, even if X-rays of the same intensity are applied to the end and central portions of the packaging bag, the transmitted X-rays are different in intensity. Therefore, it is difficult to detect a metallic contaminant with high accuracy.
There has also been proposed a technique of identifying the kind of metal by using a sensor coil. A sensor apparatus disclosed in Japanese Patent Application Post-Exam Publication No. Hei 3-18143 can identify the kind of metal passing in the neighborhood of a sensor coil by using a resonant circuit. Because the voltage output from the resonant circuit varies according to the kind of metals, e.g. copper, aluminum, and iron, the kind of metal can be distinguished. Japanese Patent Application Unexamined Publication (KOKAI) No. 2000-329858 discloses a metallic contaminant detecting apparatus having a magnetic sensor comprising a resonant circuit.
However, the conventionally proposed metallic contaminant detection using a sensor coil cannot efficiently detect small metal fragments as metallic contaminants of the order of several millimeters in size because of low detection sensitivity. It is also difficult to detect metallic contaminants in packaging bags or containers made of an electrically conductive material such as aluminum. The reason for this is that a signal generated from a packaging bag or container made of an electrically conductive material is so large that it cannot be distinguished from a signal derived from a metallic contaminant.
With the above-described technical background, the present invention was made to attain the following objects.
An object of the present invention is to provide a metallic contaminant detecting method and apparatus for detecting a metallic contaminant in a packaging bag or container made of an electrically conductive material by using a magnetic field.
Another object of the present invention is to provide a metallic contaminant detecting method and apparatus capable of detecting a small-sized metallic contaminant in a packaging bag or container made of an electrically conductive material, e.g. aluminum, with high sensitivity.
Still another object of the present invention is to provide a metallic contaminant detecting method and apparatus using a detection circuit or a signal processing circuit capable of effectively detecting a small magnetic field to detect the above-described small-sized metallic contaminant with high sensitivity.
An advantage of the metallic contaminant detecting apparatus according to the present invention resides in that a metallic contaminant can be detected with high sensitivity by setting an increased Q-value for a bridge circuit having a sensor coil, a variable-frequency power source and a variable resistor. Setting an increased Q-value causes the current in the detection circuit to have a large phase shift, so that a metallic contaminant generating a small magnetic field can be detected with high sensitivity.
Another advantage of the metallic contaminant detecting apparatus according to the present invention is the capability to detect a metallic contaminant in a packaging bag or container made of an electrically conductive material, e.g. aluminum, in particular. The resonance condition of the detection circuit can be controlled to obtain a large Q-value by adjusting the supply frequency through the variable resistor and the variable-frequency power source. Thus, it becomes possible to adjust the whole circuit smoothly.